1. Field of the Invention
The present invention generally relates to a time-varying multi-path generating apparatus, a multi-path fading simulator employing the time-varying multi-path generating apparatus, and a multi-path generating method for simulating multi-path fluctuations in radio communications systems, such as mobile communications systems; and specifically relates to a time-varying multi-path generating apparatus, a multi-path fading simulator employing the time-varying multi-path generating apparatus, and a multi-path generating method wherein time-varying multi-paths are generated by adding time-varying propagation path fluctuations to parameters such as arrival direction characteristics, phase characteristics, time delay characteristics, and power level characteristics of each propagation path corresponding to input parameters.
2. Description of the Related Art
FIG. 1 is a conceptual diagram showing the case wherein a radio propagation path is provided from a base station antenna to a mobile station, the radio propagation path often reaching the mobile station through multiple propagation paths established by reflection and diffraction due to an obstacle such as a building. On the right-hand side of FIG. 1, an example of a time delay profile is shown. In the time delay profile where five propagation paths are shown, the horizontal axis represents the propagation delay time (hereafter called time delay) of the radio propagation paths (hereafter called propagation paths) that arrive at the mobile station, and the vertical axis represents received power. The time delay and the received power may be expressed in either of absolute values and relative values.
The propagation paths (1), (2), (3), (4), and (5) shown in the delay profile are regarded as plane waves. A propagation path that has the smallest time delay is the propagation path (1) that reaches the mobile station via the shortest path from the base station, and the propagation paths with greater time delays than the propagation path (1) reach the mobile station after reflection and diffraction by buildings, mountains, etc. As for the propagation paths with the greater time delay, since the propagation loss increases as the propagation distance becomes greater with the number of times of reflection and diffraction increasing, received power tends to become smaller.
FIG. 2 is a graph that shows an example of the delay profile measured with a mobile station moving along a road (non-patent reference 1). It is clearly shown that the time delay and received power of a propagation path are changing with the movement of the mobile station.
When a mobile communications system is designed, a simulator for simulating the transmission quality of the space between the base station and the mobile station is required in order to determine whether the system can allow certain propagation path fluctuations, and in order to determine optimal system parameter values. A fading simulator is often used for evaluating the transmission quality of mobile communications systems. The fading simulator applies fading to a modulated signal that is provided by a transmitter, and outputs the faded modulated signal to a receiver. In that case, the fading simulator superimposes a modulating signal on a carrier propagation path that has a time delay, the time delay being specified by a simulator operator according to a profile model such as the Vehicular-B model (non-patent reference 2) recommended by ITU (International Telecommunication Union), etc., and applies Nakagami-Rice fading or Rayleigh fading; or, alternatively, the delay profile is calculated by ray tracing, wherein modulating signals are superimposed on carriers having different time delays, and fading that is the same as above is applied.
In conventional mobile communications systems with a relatively low transmission speed, it is sufficient to consider receiving power level characteristics (e.g., Rayleigh fading model; non-patent reference 3), and delay characteristics (e.g., equivalent transmission-line model; non-patent reference 4). For this reason, it is not necessary to generate propagation characteristics of a propagation path that properly simulates the real environment. Accordingly, fading simulators are designed to provide power level fluctuations that follow the Nakagami-Rice distribution or Rayleigh distribution that defines propagation path fluctuations as a vector sum of two or more propagation paths without taking correlations between propagation paths into consideration.
[Non-patent Reference 1]
Masaaki Shinji, “Radio propagation path propagation in wireless communications”, 1992, page 210
[Non-patent Reference 2]
ITU-R M. 1225, “Guidelines for evaluation of radio transmission technologies for IMT-2000”, 1997
[Non-patent Reference 3]
Jakes, W. C. Jr. and ed.: “Micropropagation Path Mobile Communications”, John Wiley & Sons, Inc., New York, 1974
[Non-patent Reference 4]
Y. Karasawa, T. Kuroda, and H. Iwai: “The equivalent transmission-path model, —A Tool for Analyzing error floor characteristics due to inter-symbol interference in Nakagami-Rice fading environments”, IEEE Trans. Veh. Technol. 46 [1] pp. 194-202, 1997
[Non-patent Reference 5]
Yamada, Tomisato, Matsumoto: “The system evaluation of a spatial-temporal equalizer using real propagation data” Shin-Gaku Giho A-P 2000-96 (2000-10)
[Non-patent Reference 6]
S. Ichitsubo, K. Tsunekawa, and Y. Ebine: “Multipath Propagation Model of Spatio-Temporal Dispersion Observed at Base Station in Urban Areas”, IEEE Journal on selected areas in communications, Vol. 20, No. 6, August 2002.
[Problem(s) to be Solved by the Invention]
Indispensable technology for the 4G (fourth generation) mobile communications (broadband-transmission) includes adaptive equalization technology for removing an unnecessary propagation path on the delay time axis, and adaptive array antenna (AAA) technology for separating an unnecessary propagation path on the space axis (the radio propagation path arrival direction). Research and development towards the realization of these technologies are being energetically furthered (non-patent reference 5). Further, where the transmission speed (bit rate) is high, an incoming propagation path can be separated into multiple propagation-delay propagation paths (a single propagation path, or multiple propagation paths compounded). For this reason, the influence of fading that is a problem in conventional transmission systems at a low transmission speed can be mitigated in high-speed transmission, and communication quality can be improved. Accordingly, in order to perform highly precise evaluation of transmission characteristics, it becomes important that propagation characteristics be individually reproducible for each of such multiple propagation paths between the base station and the mobile receiving station.
The propagation characteristics of a propagation path to be considered include propagation-delay characteristics and arrival direction characteristics, and analysis of the characteristics and modeling thereof are being studied (non-patent reference 6). For example, it is known that an exponential-function model reasonably approximates the propagation-delay characteristics, namely, the received power exponentially decreases according to the time delay (non-patent reference 3). However, in model approaches such as described above, the time variation characteristics of the propagation path are defined as a mere superimposition of either the Nakagami-Rice fading or the Rayleigh fading fluctuations, where fluctuation characteristics of the multiple propagation paths are simply added.
Furthermore, in order to evaluate a communications system using adaptive equalization technology and AAA technology, a propagation path model needs to consider all of the arrival direction characteristics, delay time characteristics, and receiving power level characteristics.
As a factor of the power level fluctuation of each propagation path, shadowing (blocking) of the propagation path by the presence and movement of an obstacle such as a car and a human body is to be considered. In addition, when the mobile station moves, a building and the like may interfere with the propagation path. For this reason, evaluation of each propagation path has to take the power level fluctuation caused by these obstacles into consideration; however, conventional fading simulators do not consider these matters.
Further, as for the propagation paths received by the mobile station in directions that are approximately the same, there must be meaningful correlation between the propagation paths, since the same obstacle would interfere with each of the propagation paths. However, a fading simulator that considers the correlation characteristics of such propagation paths is not yet offered.